The rapid ascent of graphene has driven extensive interest in additional atomically thin elemental two-dimensional (2D) materials including phosphorene, stanene, and most recently, borophene. (See, A. J. Mannix, X.-F. Zhou, B. Kiraly, J. D. Wood, D. Alducin, B. D. Myers, X. Liu, B. L. Fisher, U. Santiago, J. R. Guest, M. J. Yacaman, A. Ponce, A. R. Oganov, M. C. Hersam, N. P. Guisinger, Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs, Science 350, 1513-1516 (2015)). Unlike the naturally layered structures of bulk graphite and black phosphorus, boron exhibits significantly more complex and diverse bulk structures due to the rich bonding configurations among boron atoms. Studies of atomically thin boron sheets (i.e., borophene) primarily relied on theoretical predictions until recent studies experimentally demonstrated borophene synthesis on Ag(111) substrates. These experimental studies have confirmed theoretical predictions that borophene is a 2D metal and can adopt multiple structurally distinct phases as a function of processing conditions.
As an emerging 2D material, borophene has thus far only been studied in isolation, whereas nearly all technological applications will require the integration of borophene with other materials. Of particular interest are electronically abrupt lateral heterostructures, which have been widely explored in other 2D materials due to their novel electronic properties. For example, atomically well-defined lateral heterostructures between graphene and hexagonal boron nitride have revealed spatially confined boundary states with scanning tunneling spectroscopy (STS). It should be noted, however, that methods for experimentally realizing atomically clean and abrupt lateral heterojunctions remain challenging for many 2D material systems. For example, the growth front of the first 2D material can be easily contaminated, which can disrupt the subsequent growth of the second 2D material and/or lead to ill-defined interfacial regions. Alloying and intermixing during the growth of 2D material lateral heterostructures also prevents abrupt interfaces.